The present invention relates to determining the resistivity of geological formations surrounding a well that is provided with a metal casing.
The importance of resistivity logs in oil prospecting is well known. It is known that the resistivity of a formation depends essentially on the fluid it contains. A formation containing salt water, which is conductive, has resistivity that is much lower than a formation filled with hydrocarbons, and consequently resistivity measurements are of irreplaceable value for locating hydrocarbon deposits. Resistivity logs have been made very widely and for a long time, particularly by means of devices having electrodes, however existing techniques have a field of application which is limited to wells that are not cased (known as xe2x80x9copen holesxe2x80x9d in oil industry terminology). The presence of metal casing in a well, where the resistivity of metal is tiny compared with values that are typical for geological formations (about 2xc3x9710xe2x88x927 ohm.m for steel casing compared with 1 to 100 ohm.cm for a formation), represents a considerable barrier to sending electrical currents into the formation surrounding the casing. As a result, it is essential for resistivity measurements to be performed before the casing is put into place. In particular, resistivity measurements cannot be obtained from wells that are in production since they are fitted with casing.
It would therefore be most advantageous to be able to measure resistivity in cased sections of wells. Such measurement, performed in a well that is in production and at the level of the deposit, would make it possible to locate the water-hydrocarbon interfaces, and thus to track the positions of such interfaces over time, in order to monitor the behavior of the hydrocarbon deposit and optimize exploitation thereof. It would also be possible to obtain resistivity measurements in a well (or a section of well) where no measurements were performed prior to the casing being put into place, in particular to improve knowledge about the deposit, and perhaps find productive layers that were not located initially.
Proposals on this topic are to be found in the literature. The principle on which such measurements are based, described in U.S. Pat. No. 2,459,196, consists in causing a current to flow along the casing under conditions in which current leaks out or is lost to the formation. This loss is a function of the resistivity of the formation, the more conductive the formation the greater the loss, thus by measuring the loss it is possible to determine the resistivity of the formation. According to the above-mentioned patent, current loss is evaluated by establishing a profile for the current flowing along the casing. U.S. Pat. No. 2,729,784 describes a measurement method that uses three measurement electrodes spaced apart along the casing and forming pairs of adjacent electrodes that are theoretically identical. Current electrodes are placed on either side on the measurement electrodes to inject currents in opposite directions into the casing. A feedback loop servo-controls current injection so as to put the outer measurement electrodes at the same potential for the purpose of eliminating the effect of differences in the resistance of the casing in the sections defined by the measurement electrodes. A value for the leakage current at the middle electrode is obtained by measuring the voltage drop in each of the pairs of electrodes and by taking the difference between the voltage drops, said difference being stated to be proportional to the leakage current. U.S. Pat. No. 2,891,215 describes a method of the same type using an additional current electrode level with the middle measurement electrode, and disposed so as to apply a current which exactly compensates the leakage current.
As in U.S. Pat. No. 2,729,784, French patent 2 207 278 provides for the use of three regularly-spaced measurement electrodes to measure current leakage, and it describes a two-stage method: a first stage for measuring the resistance of the section of casing defined by the measurement electrodes, during which stage the current is caused to flow along the casing so that there is no leakage into the formation; and a second stage during which a current leak can take place into the formation. To this end, a current injection system is provided that comprises an emitter electrode and two return electrodes, one close to the measurement electrodes and active during the first stage, and the other situated on the surface, and active during the second stage.
U.S. Pat. No. 4,796,186 describes a two-stage method of the same type as above-mentioned French patent 2 207 278, and it uses the same disposition of electrodes. It provides a circuit for eliminating the effect of variations in resistance between the two sections of casing, which circuit comprises amplifiers connected to each pair of measurement electrodes so as to deliver respective output voltage drops. One of the amplifiers has variable gain, and its gain is adjusted during the first stage so as to cancel the difference between the outputs from the amplifiers. That technique is very difficult to implement, given the orders of magnitude specified above. It also requires two distinct measurement stages.
The invention seeks to enable leakage current to be determined in a manner that is simpler and more effective than in the known techniques.
The invention provides a method of studying the resistivity of a geological formation around a borehole fitted with metal casing, the method being characterized by the fact that an electric current is applied to the casing so as to cause current to leak into the formation at a given level, said current is shunted by a feedback circuit in contact with the casing on either side of the measurement level, said feedback circuit being organized so that the current flowing along the casing at said level is small compared with the shunt current, the difference between the voltage drops on adjacent sections of casing situated on either side of the measurement level is determined, and the leakage current is deduced therefrom.